1. Field Of The Invention
This invention relates to a coating apparatus for applying a coating onto a surface of a moving web. More particularly, this invention relates to a short dwell coating apparatus for coating a surface of a paper web.
2. Information Disclosure Statement
In recent years, much research has been carried out in an effort to efficiently coat a surface of a paper web with coating material. In the prior art, various methods have been employed for applying such coating material including feeding a web around a backing roll, precoated by an application roll for applying coating to the web. With most of these methods, a blade disposed downstream relative to the coating material pond urges the coated web against the backing roll for removing excess coating material from the web.
More recently, the so-called short dwell coater has been introduced which avoids the problems associated with an open pond of coating material. The short dwell coater enables coating material under hydraulic pressure to be applied to a limited area of a moving web with a flexible blade defining the downstream edge of such area. Any excess coating material flows from such area away from the blade in a direction opposite to the direction of movement of the web. Among the advantages provided by such short dwell coaters may be included the short time that the coating material is disposed in contact with the web prior to removal of excess coating material by the blade. Such short time period inhibits absorption of the coating material throughout the web. By use of such short dwell coater, less water is forced into the web thereby resulting in a stronger web at the blade. Because the short dwell coater technique is more of a surface application, there is a decrease in blade pressure and consequently, a reduction in the number of web breaks which occur during the coating operation.
The short dwell coater presents many advantages over the prior art coating methods. However, a problem exists with the short dwell coater when such coater is used to coat a web moving at high speed. When the web is moving in the region of 3,000 or more feet per minute past the short dwell coater, there is a tendency for an air pocket to develop between the backing roll and the web upstream relative to the short dwell coater. Because of the relatively short period of time that the coating material is in contact with the web prior to excess coating material being removed by the blade, the aforementioned air pocket, when coating at high speeds has sometimes resulted in web breakage and streaking. Furthermore, there exists a tendency for any particles of contamation on the surface of the web or in the short dwell coating pond to become lodged against the blade thereby causing marking of the surface of the coating and sheet breaks.
The present invention overcomes the aforementioned problems by the provision of a backing roll having a perforate surface with a permeable belt disposed between the perforate surface and the web. When a vacuum is applied to the backing member, the web is drawn into close conformity with the belt thereby inhibiting the generation of an air pocket between the web and the backing member and preventing streaking, lodging or hangup of deleterious material adjacent to the blade.
The primary objective of the present invention is the provision of a coating apparatus that overcomes the aforementioned problems associated with the prior art coating apparatus and that provides a significant contribution to the art of coating moving webs.
Another object of the present invention is the provision of a coating apparatus including a backing member defining a perforate surface. The coating apparatus further includes a permeable belt extending around the backing member and disposed between the web and the backing member such that when a source of partial vacuum is connected to the backing member, air flows through the permeable belt and the perforate surface for drawing the web into close conformity with the belt so that the belt and web move together and in the same direction for inhibiting the buildup of deleterious material adjacent to a coating blade.
Another object of the present invention is the provision of a coating apparatus in which the permeable means is a wire mesh belt which includes a first layer disposed contiguous relative to the perforate surface and a second layer disposed contiguous relative to the first layer with the second layer being disposed contiguous relative to the web. The second layer is of a finer mesh relative to the mesh of the first layer such that during coating of the web, vacuum applied through the perforate surface is evenly distributed by means of the first layer and the second layer inhibits marking of the contiguous surface of the web.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the detailed description set forth hereinafter taken in conjunction with the annexed drawings and the appended claims.